Processes for preparing polyolefins using transition metal compounds such as metallocene compounds have been recently paid much attention. The transition metal compounds are characterized in that when they are used as a catalyst component for olefin polymerization, they exhibit a high polymerization activity and the resulting polymer has a narrow molecular weight distribution.
Meanwhile, in polyolefins which are obtained by the use of such transition metal compounds as mentioned above, the transition metal compounds used as the catalyst component are contained. Ligands of the transition metal compounds are residual groups of cyclic compounds having conjugated double bond of cyclopentadienyl skeleton, so that they sometimes become sources of odor development when they are thermally processed, and besides the odor development may have bad influences on flavor, etc. in a field of foods where delicate smell or taste is considered as important. Therefore, the polyolefins obtained by the use of the transition metal compounds are sometimes restricted in their uses.
As a method of treating resins to inhibit odor development of the resins in the molding process, for example, a method of drying resin pellets over an inert gas to remove the ligands has been proposed in Japanese Patent Laid-Open Publication No. 157486/1975, or a method of treating resin pellets with a hot water column to remove the ligands has been proposed in Japanese Patent Publication No. 18521/1982.
By the conventional technique, however, odor development cannot be sufficiently inhibited because of insufficient removal of the ligands, or the removal of the ligands needs much time or large energy.
Under such circumstances as described above, the present inventors have earnestly studied, and as a result, they have found that the ligands, which are sources of odor development, can be efficiently removed by the method comprising the steps of contacting polyolefin with a ligand decomposer such as water, alcohol or the like to decompose the residual ligands contained in the polyolefin and heating the polyolefin contacted with the ligand decomposer. Based on the finding, the present invention has been accomplished.